A View of the Map
Understanding How the Marauder's Map Works

But first, let's look at the
castle. If you research castles on
the Internet, you will see that
Hogwarts is a castle of unprecedented
(mammoth, huge, gigantic) size. You might find some castles that have towers
that are 7 commercial stories high, but you will not find castles that have 7
floor main buildings. Example, a Keep, which is an older style square
tower-like castle, is typically 90 feet high and will have three or at the
most four floors in it. Hogwarts, in
my imaginative view, has several wings all 7 floors high with very high
ceilings, and its many towers are much much higher than that. Given that
castles always have very high ceilings
[Actually, except in special-purpose rooms such as great halls and
chapels, the opposite tends to be true for most areas within a castle.
- ed], I can picture the main 7 floor wings
of Hogwarts being as high as a
12 to 14 story commercial or residential building. That would make the high
towers in the range of a 20+ floor high common building. That is
ONE BIG MOTHER of a castle.

Now picture that castle's complete
floor plans drawn in standard architectural format. That would be many many
extremely large documents. But the
Marauder's Map is
a single piece of parchment. Even if it is 3 feet on a side, the floor plans
plus the castle grounds
would have to be drawn at microscopic size to fit in that space.

I've always had the idea that the castle floor plans were layered on
top of each other, and when you wanted to see a specific floor it came
into sharp relief while the remaining overlapping floors fade to a pale
unobtrusive grey. Even if that speculation is correct, the drawing would
still have to be very tiny.

Plus, in combination with shifts in the wizard's conscious desire, it
could be very finely attuned to the mind of the user, to his subconscious
fears and concerns, and could adjust what it shows based on the slightest
shift in the perception, or mental state of that user. Just as your eyes
make constant subtle shifts in focus, the map could shift what it shows
based on subtle shifts in your mental focus; conscious or unconscious mental
focus.

The minuscule size of the drawings alone could easily explain why
Fred and George never saw
Peter/Wormtail, and why
Lupin never saw the second
Harry and
Hermione on
the map during
their time travels. So, the only thing you see clearly is the tiny area you
are intensely concentrating on. Everything else fades to a blur.

Selective Display

Next, many have speculated that in order to keep
the map from being
too cluttered to use, it is selective in what it shows. As far as
the map being
selective, there are many different ways in which it could be selective.

Another possible reason why some areas wouldn't be shown is that it's
possible that there are areas on
the Map
that are irrelevant. Since
the map
is for 'mischief making', there would be no need to know who was in the
Great Hall or the
common rooms. It might be nice to know
Snape was in the
Great Hall, but
unless you are making mischief in the vicinity, it serves no purpose. Plus
the Great Hall and
the common rooms are very crowded, so
the map would
show one big jumble of indecipherable names.

Which brings us to the next reason why some areas wouldn't be shown. It
is possible that it doesn't show extremely crowded areas, like the common
rooms. Showing the common rooms would be especially ineffective at certain
times of the day when most of the students are there studying. Again, all
you would see is a hopeless jumble of names.

Another mode of selective display could be proximity. You don't need
to know if Snape is in the furthermost
point of the castle, but you would certainly like to know if he were nearby.
So, picture an invisible bubble of importance surrounding the user. Anything
within the boundaries of this bubble would be highlighted on
the map. This
wouldn't be so much a matter of 'live' spots or 'dead' spots, but more a
matter of highlighted foreground with the irrelevant faded into the
background.

Another explanation of how so much information could be contained on such a
small map is MAGIC. Perhaps the map is like Moody's seven-key trunk
or the boot (trunk) of Mr. Weasley's car. The surface area of
the map is far
larger than its perimeter dimension. As you scan the surface of
the map, your
visual field grows while the size of the parchment remains fixed. Even if this
is true, for it to be visually functional, the size of the drawings would
still have to be small, and Harry
confirms this. Harry constantly refers to
everything on the map
as minute (as in tiny, not 60 seconds).

Another 'dead' spot is the forest.
The map can't possibly show all of the castle, the grounds, and the
forest too. So, I speculate that
where the forest is shown,
the map only
penetrates 10 to 15 feet. That explains why
Barty Crouch Sr. wasn't seen until he
moved to the edge of the forest
near Harry and
Krum.

I'm sure other people can came more ways in which the map could be selective.

Potential Conflicts in the Story

For the moment, let's say it is not selective. Now picture viewing
Gryffindor tower on
the map, what are you going to see? A hopelessly undecipherable
jumble of overlapping names. That would certainly make it difficult to
pick Peter Pettigrew out of the crowd.

Next, Fred and George are at school; the
context of the situation controls how they will interpret what they see. In
the context of school, seeing the name of some underclassman
(Peter P. or
P. Pettigrew) in close proximity to
Ron would mean nothing to them. In that
context, where they are conditioned to expect to see the names of
teachers and kids. I
seriously doubt that one of them would see the name and go,
'Hey, isn't that the obscure old dead guy that we have some vague knowledge
of?". I don't think so.

Then of course, they used the map for mischief making, and would have
no reason to look for anyone other than someone who could affect that
mischief making. Again, context controls what they see.

Well, he wasn't looking for them. He was specifically looking for
Harry,
Ron, and
Hermione to go down to
Hagrid's. So he concentrated
on the route from the castle to
Hagrid's.
Time travelling Harry and
Hermione were only on the
grounds for a brief
run, and by that time, Lupin had already
spotted the 'normal' Harry and
Hermione. After that,
time travelling Harry and
Hermione spent most of their
time hidden in the forest, and
in all probability, off the map. When they exited the
forest to get
Buckbeak,
Lupin was concentrating on
Harry, Ron, Hermione, and Scabbers walking back to the castle, and was
then distracted by the attack by Sirius, the dog, after which time, he
was rushing to their aid. He would have had no reason to search the map fully.

Barty Crouch searches Snape's Office

In the Yahoo HPforGrownups group, Linda pointed out:

"A single dot was flitting around a room in the bottom left
corner --- Snape's office. But the dot wasn't labeled 'Severus Snape'...it
was Barty Crouch." (GF25)

She then asked a question related to the Map's selective viewing. She
pointed out that Harry saw Barty Crouch but he would have had no desire,
need, or intent to see Crouch. Harry would have no reason to be looking
for Barty Crouch. He thinks he is at home, too ill to even go to work.
He wasn't looking to see if Snape was in his office because the preceding
text line simply says that the movement caught his eye, not that he checked
to see if Snape was in his office.

and I responded-

Barty would not be of interest to Harry in that he would never
think to look for Barty on the map, and it may have been the movement that
caught his eye, but I don't think that shoots down the original theory.

The key is not Barty or Snape, but
Snape's office.
Snape's office is
an area that Harry would routinely and habitually be concerned with. It
would be an area of interest to him on the map. So, I think it is safe
to assume that this would be part of a default set of areas that would
show up on the map.
Snape's office,
Filch's office,
Dumbledore's office,
as well as their quarters, and the hallways would all be areas of concern
to any rule breaker because they represent some of the people most likely
to be out trying to catch rule breakers.

You are right, it was movement that caught Harry's eye, but more than that it
was movement in
Snape's office,
and Harry would certainly be interested in any activity in there.

Final conclusion

Even if the map is aided by magical size distortion, everything has
be very, very, very small, and available information has to be limited
by some relevance, and therefore, the map takes forced concentration, focus
and effort to view. This size, focus, and effort would reduce or eliminate
the likelihood of accidentally viewing random peripheral information.